THE human race owes its susceptibility to many diseases to widespread inbreeding early in our evolutionary history, new research has suggested.

Analysis of the human and chimpanzee genomes by British scientists has revealed that damaging genetic defects accumulated in both species because both are descended from a common ancestor that existed in very small numbers.

The small population of “missing link” apes, from which people and chimps ultimately evolved, effectively made it impossible for natural selection to weed out potentially dangerous genetic mutations.

This has left both species with many more errors in key sections of the genome than mice or rats. Humans and chimps have inherited “sloppy” systems for controlling how genes are switched on and off, which makes us more susceptible to diseases such as cancer, according to the researchers.

The findings add weight to the now well-supported theory that the peculiar genetic characteristics of different species often develop because of “evolutionary bottlenecks” in which only small numbers survive.

Populations become inbred as a result, and any deleterious random genetic mutations, which would normally die out as a result of normal natural selection, stand a good chance of staying in place.

In the new research, scientists from the University of Edinburgh, the University of Bath and the University of Sussex compared the human and chimpanzee genomes with those of rats and mice.

They found that since the last common ancestor of humans and chimps existed about six to seven million years ago, approximately 140,000 naturally-occurring mutations have emerged in key parts of their descendents’ genomes.

This compares unfavourably with mice and rats, which have much more effective “gene control” mechanisms, that govern when genes are switched on and off.

The results, which are published today in the journal Public Library of Science Biology, suggest that the ancestors of humans and chimpanzees passed through an evolutionary bottleneck during which as few as 10,000 individuals survived.

Mice and rats did not undergo this process: they have evolved from a much larger ancestral population, and thus mutations were rapidly eliminated from the gene pool.

Martin Lercher, of the University of Bath, said that the discovery indicates that humans are anything but an evolutionary triumph, as people often like to assume. “We are used to viewing us as the pinnacle of evolution but seeing that rodents control their genes much more precisely is somewhat sobering,” he said.

As well as the ancient population squeeze that affected the common ancestors of humans and chimps, Homo sapiens is also thought to have passed through a more recent bottleneck. There is considerable genetic evidence that everyone alive today is descended from just a few thousand individuals who survived in Africa between 50,000 and 100,000 years ago.

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